This invention relates to an optical glass having optical constants of a refractive index (nd) within a range from 1.55 to less than 1.67 and an Abbe number (νd) within a range from 30 to 45.
As optical glasses having optical constants within the above described ranges, the Glass Composition Data Book 1991 published by Japan Glass Product Industry Association disclose compositions of barium flint glass (BaF4) and heavy barium flint glass (BaSF3, BaSF4 and BaSF6). These glasses have the advantage that the cost of raw materials is relatively low but contain PbO and As2O3 which cause the environmental pollution and waste glass and sludge which are produced when these glasses are processed pollute the environment. Besides, since PbO is an ingredient which increases specific gravity of the glass, the weight of an optical instrument increases if such glass is used for a lens of the optical instrument. For these reasons, there is a great demand for an optical glass which is free of PbO and As2O3. Since, however, PbO among these ingredients has a great influence on optical and chemical properties of glass, it has been difficult to develop an optical glass which has optical and chemical properties which are equivalent to the glass containing PbO and still is free of PbO.
As optical glasses having optical constants within the above described ranges and being free of PbO and As2O3, Japanese Patent Application Laid-open Publication No. 52-25812 discloses an optical glass of a SiO2—Nb2O5—TiO2—R2O system. Japanese Patent Application Laid-open Publication No. 52-45612 discloses an optical glass of a SiO2—Nb2O5—R2O—RO system. Japanese Patent Application Laid-open Publication No. 6-107425 discloses an optical glass of a SiO2—B2O3—TiO2—Nb2O5—BaO—R′2O system. These glasses, however, have the following disadvantages:
(1) Since these glasses contain Nb2O5 which is a very expensive ingredient as an essential ingredient, the cost of raw materials of the glass becomes high.
(2) In a case where raw materials of the glass are melted in a large scale production, the raw materials are put in a crucible in several divided parts. Since Nb2O5 is a bulky material, the glass containing Nb2O5 must be put in the crucible in a larger number of divided parts than a glass which does not contain Nb2O5 even if the melting property of these glasses is the same. Thus the glass containing Nb2O5 requires longer total time for placing the raw materials in the crucible with resulting increase in the manufacturing cost.
In the glasses of Japanese Patent Application Laid-open Publication No. 52-25812 and Japanese Patent Application Laid-open Publication No. 52-45612, a part of Nb2O5 may be substituted by Ta2O5. Since, however, Ta2O5 is more expensive than Nb2O5, the disadvantage of the high cost cannot be eliminated even if such substitution is made.
Costs of raw materials usually vary with lapse of time but the costs of Ta2O5 and Nb2O5 are permanently high. Since the cost of raw materials of the barium flint glass and heavy barium flint glass which contain PbO and As2O3 is low, the cost of glass products using these glasses is also low. For this reason, there is a strong demand by users such as manufacturers of optical instruments for an optical glass which has the above described optical constants and does not contain PbO or As2O3 which is detrimental to the environment and moreover is low in the cost of the raw materials and therefore low in the cost of the resulting products.
It is, therefore, an object of the present invention to provide an optical glass which has eliminated the above described disadvantages of the prior art optical glasses and has optical constants of a refractive index (nd) within a range from 1.55 to less than 1.67 and an Abbe number within a range from 30 to 45, is free of ingredients such as PbO which is detrimental to the environment and also is free of expensive ingredients such as Ta2O5 and Nb2O5 and, therefore, has little likelihood of the environmental pollution and can be manufactured at a low cost.